1. Field of the Invention
The present invention generally relates to apparatuses and methods to restore or prevent erosion of shorelines and beaches. More particularly, the present invention relates to an apparatus and method for shoreline reclamation that uses a plurality of stanchions and a compliant porous barrier fastened to the stanchions to create a temporary structure that is placed in the water flow, proximate to the shoreline, and the structure causes accretion of sediment suspended in the water flow.
2. Description of the Related Art
Shorelines on bodies of moving water, such as rivers and oceans, will erode from natural processes removing material from the shoreline. This erosive process is sometimes referred to as xe2x80x9cscour,xe2x80x9d and the natural processes of movement of material along a coastal shoreline are referred to as littoral processes. In scour, the moving water suspends the material at one location in the flowing water and then redeposits the material at some other location. Many factors specific to the particular shoreline and water velocities can enhance the erosion phenomenon.
One significant factor is the consistency of the material comprising the shoreline. A sandy beach is easily eroded by a slow and steady stream of water, and can be quickly eroded in very turbulent and fast moving water such as the seas associated with a major storm. Conversely, shoreline comprised of mostly rocks or larger sediment will be much less susceptible to erosion.
Another significant factor enhancing the erosion process is the velocity of the water passing across the shoreline. In order to initiate scour, the water must move at a velocity greater than a critical xe2x80x9csuspension velocityxe2x80x9d to suspend the sediment of the shoreline in the moving water. The suspension velocity required to initiate scour is dependent upon many location specific factors, such as the geometric shape of the shoreline, the average velocity of the water, the average direction of flow of the water in relation to the shoreline, the depth of the water, the density of the sediment material to be transported.
Shoreline erosion is a serious problem because most of the urban areas of the world are ports having urban development right up to the shoreline. There are often structural improvements present at and near the shoreline, such as private beach homes, hotels, bridges, retaining structures, and the like, and shoreline erosion progressively undermines the foundations thereof and threatens the physical integrity of the structures over time. There are also many regions with beach tourism as their main industry, and thus, beach erosion can cause these regions significant economic harm by removing the main tourist attraction.
There have been many devices and methods of hydraulic and earth engineering employed in the attempt to preserve shorelines or other areas subject to the erosive influence of moving water. The main method of combating erosion is to simply renourish an eroding beach with a fresh supply of dredged sand. This method has many problems associated with it however. The dredged sand often does not match the existing color of sand on the beach and diminishes the aesthetic appearance of the beach. The dredged sand can also contain rocks or other solid objects that can hinder water sports such as swimming or surfing, and can hurt the bare feet of waders upon the renourished beach.
Other methods to prevent shoreline erosion fortify the eroding shoreline with blocks, cement and the like so as to form a prophylactic layer over the region of the shoreline that would otherwise be subject to the erosive effects of the moving water. However, due to the weight and bulk of the fortifying materials, such xe2x80x9carmoringxe2x80x9d techniques are often difficult to install on the shoreline and adequately anchor the armor to the underlying shoreline, whether beach, bank or both. The armored structures often result in permanent structures that are not easily removed from the shoreline and prevent full enjoyment of the region of the shoreline that they overlay.
Jetties or groins are also known for attempting to control shoreline erosion. As is well known to those skilled in the art, each shoreline has a natural water direction and flow rate in accord with which it migrates. In the typical construction, a jetty of stone or other permanent formation is built into the shore so as to form a jetty traverse the natural flow direction of the shoreline. While the jetty has the advantageous effect of promoting local sediment deposition, the jetty has a distinct disadvantage in that it causes downstream and upstream erosion. And if too many jetties are installed along a given region of shoreline, the jetties may alter the dynamic equilibrium of the shoreline and undesirably change the shape of the beach as a whole, especially when the shoreline is subject to a significant erosive event such as a storm or flood.
There are other shore and bank protection techniques and devices known in the art that attempt to control erosion by attenuating the energy, velocity, and/or direction of a potentially erosive water flow with the use of temporary structures placed on the shoreline. Several of these devices are porous groins structures using either flexible or rigid nets, screens, or filters placed on the shoreline substantially perpendicularly to the shoreline and extending into the surf. The porous groins are placed in the tidal and longshore currents and function in much the same way as a jetty to cause sand to accrete around the porous groin. The porous groin must be constantly moved or removed from the accreting sand or else extreme force must be used to dislodge the porous groin from the accreted sediment.
Moreover, many of these structures cite their success in beach restoration as arising from the ability of the net, screen, or filter to trap larger sediment being pulled along the sea bottom to cause ridges to build-up at the base of the porous groin. However, these structures have also been used to successfully restore pure-sand beaches, i.e. where larger sediment, such as rocks, coral, shells, and the like are not significantly present in the sediment comprising the beach. Thus, the extant explanation for success of these devices is unsatisfactory given the success in restoration of pure-sand or sediment shorelines.
Accordingly, it would be advantageous to provide a device and method for shoreline restoration that uses temporary structures to renourish the beach taking full advantage of the correct mechanism for the accretion of sand and sediment from the eroding water flow. Such device and method should renourish the beach without adversely altering the surrounding shoreline. It is thus to such a shoreline reclamation device and method that the present invention is primarily directed.
The present inventive system and method provides a compliant porous groin for restoring an eroding shoreline utilizing the particular accretion mechanism for a water flow that contains suspended sediments. The water flow has a critical accretion velocity as it flows across the eroding shoreline, and if the water flow velocity is slower than a critical accretion velocity above which sediments otherwise remain suspended in the water flow, the suspended sediments will accrete from the water flow. The compliant porous groin takes advantage of this mechanism to renourish the sediment of an eroding shoreline, such as sand on a beach.
The compliant porous groin comprises at least two supports placed in the eroding shoreline with a compliant porous barrier attached to the supports such that the barrier is at least partially within the water flow of the shoreline and the water flow passes through at least a portion of the barrier. The supports are any rigid or semi-rigid structure that can support the barrier in the water flow, such as a stanchion, tripod, pole, or channel. The barrier is compliant such that the sediment-laden water flow impacting the solid portions of the barrier is slowed to at least the critical accretion velocity such that the sediment accretes from the water flow adjacent to the barrier.
The shoreline includes a beach portion that does not ordinarily have water upon it, a substantially water-covered portion, such as an inter-tidal region, and the water portion, generally below the low-tide line. The at least two supports can be placed entirely in the substantially water-covered portion, with at least one support in the beach portion and at least one support in the substantially water-covered portion, or with at least both supports in the water portion outside of the low-tide line.
The actual compliance of the barrier can be achieved through several methods. The barrier can be made of a rigid material, such as rigid plastic webbing or wire mesh, and be flexibly held to the support to be compliant to the impacting eroding water flow. Alternatively, the barrier can made of an elastic material, such as semi-rigid plastic webbing, a mesh (organic or polymer netting), or other interwoven series of members that are compliant to the impacting eroding water flow.
The invention further provides a method for restoring a shoreline having a eroding water flow moving at a velocity thereacross with suspended sediments therein and having a critical accretion velocity wherein the suspended sediments accrete from the water flow if the velocity of water flow is less than the critical accretion velocity, the method including the steps of placing at least two supports in the eroding shoreline, attaching a compliant porous barrier to the at least two supports such that the barrier is at least partially within the water flow of the shoreline and the water flow passes through at least a portion of the barrier, and accreting sediment from the water flow with the compliance of the barrier slowing the water flow impacting the barrier to at least the critical accretion velocity. The method preferably further includes the steps of lifting the barrier out from the accreting sediment as sediment accretes from the water flow to cover the barrier, and removing the barrier and supports from the shoreline after the shoreline has been renourished.
If the step of attaching a compliant porous barrier to the at least two supports is attaching a rigid porous barrier to the at least two supports such that the barrier is flexibly held to the at least two supports, then the step of accreting sediment from the water flow impacting the barrier is accreting sediment from the water flow impacting the rigid barrier made compliant to the impacting water flow from the flexible attachment of the barrier to the at least two supports, the compliance of the rigid barrier slowing the impacting water flow to at least the critical accretion velocity. And if the step of attaching a compliant porous barrier to the at least two supports is attaching an elastic porous barrier to the at least two supports, then the step of accreting sediment from the water flow impacting the barrier is accreting sediment from the water flow impacting the elastic barrier that is compliant to the impacting water flow from the elasticity of the barrier, the compliance of the elastic barrier slowing the impacting water flow to at least the critical accretion velocity.
The compliant porous groin thus advantageously performs shoreline restoration using the compliance of the barrier to effect the accretion of sand and sediment from an eroding water flow. The accretion can be optimized as the compliance of the barrier can be adjusted to specifically offset a given water flow such that the impacting sediment-laden water will be slowed to at least the critical accretion velocity. The compliant porous groin does not significantly interfere with the longshore transport such that its use adversely alters the shoreline surrounding the renourished area. Further, the compliant porous groin is a temporary structure that can be used to renourish the beach and be removed thereafter with almost no environmental impact. It is thus to such a shoreline reclamation device and method that the present invention is primarily directed.
Other objects, advantages, and features of the present invention will become apparent after review of the hereinafter set forth Brief Description of the Drawings, Detailed Description of the Invention, and the Claims